Dispersion strengthened metals

ABSTRACT

A process for dispersion strengthing of atomized alloy particles by internal oxidation is substantially improved by providing atomized particles substantially free of oxide surface film to enhance the efficiency of the internal oxidation step. The atomized alloy particles can be produced by dry collection to prevent oxide film formation, or alternatively, the oxide surface film can be removed by mechanical action removal or by chemical leaching.

BACKGROUND OF THE INVENTION

This invention pertains to an improved process for producing improveddispersion strengthened alloy metal products.

Dispersion strengthened metal products, such as copper dispersionstrengthened with aluminum oxide, have many commercial and industrialuses. Welding electrodes, for example, require good electrical andthermal conductivities together with good strength and hardness atelevated temperatures. Dispersion strengthening has been recognized inthe past as a method for increasing strength and hardness of metals. Asolid solution alloy comprising a relatively noble matrix metal havingrelatively low heat or free energy of oxide formation and a solute metalhaving relatively high negative heat or free energy of oxide formationcan be dispersion strengthened by heating the alloy under oxidizingconditions to preferentially oxidize the solute metal. This technique ofoxidizing the solute metal to a solute metal oxide is known in the artas in situ internal oxidation or more simply internal oxidation.

Several processes for internal oxidation have been suggested. Commonlyassigned U.S. Pat. No. 3,779,714, incorporated herein by reference,provides an improved alloy-oxidant mixture wherein the oxidant includesan oxide which releases oxygen to oxidize the solute metal of the alloy.A further improvement is set forth in commonly assigned U.S. Pat. No.3,893,844 which provides improved dispersion strengthened metals byfirst recrystallizing the alloy powder prior to internal oxidation toincrease the grain size of the alloy to a grain size at least as largeas Grain Size No. 6 as measured by ASTM Test. No. E-112.

In commonly assigned U.S. Pat. No. 3,779,714, a dilute solid solutionalloy powder is obtained by atomizing an alloy melt of matrix metal witha minor amount of solute metal wherein the melt is atomized by nitrogenand collected as powder in water. During the atomization process,however, oxygen in the atomization chamber tends to oxidize the solutemetal on the surface of the alloy particles. For instance, acopper-aluminum alloy tends to become oxidized during atomization toform an aluminum oxide film on the particle surfaces. Further oxidationof the surface aluminum can occur upon the powder contacting water inthe collection tank. The hot powder falling into the water generatessteam in the atomization chamber which further contributes to thesurface oxidation of the alloy particles. The accumulative effect ofsurface oxidation is the formation of a relatively thick aluminum oxidefilm on the surface of the alloy particles.

It now has been found that the surface oxide film formed on the alloyparticles remains intact and can be detrimental in subsequent internaloxidation by forming a barrier to internal oxidation. Subsequentprocessing can inhibit interparticle bonding during subsequentfabrication of the powder into compacted fully dense parts obtained, forexample, by hot forging and rolling. The final product can be weak andbrittle. In a subsequent hot extrusion step, the powder particles tendto stretch out into elongated fibers thus improving interparticlebonding, although surface oxide film has been found to remain on fiberinterfaces and causes diminished mechanical properties due to improperand incomplete interparticle or interfiber bonding. Elimination of thesurface oxide has been found to substantially improve the mechanicalproperties of the dispersion strengthened alloy such as stress rupturestrength as well as substantially improve the internal oxidation stepfor dispersion strengthening alloys. The surface oxide forms a barrierto the diffusion of oxygen into the alloy particles and, therefore,elimination of the surface oxide provides efficient, uniform, andeffective oxidation of higher solute metal alloys. Significantimprovements in the properties of internally oxidized alloys can beachieved by atomizing alloys in helium and collecting the alloy powderdry. These and other advantages will become more apparent by referringto the Detailed Description of the Invention.

SUMMARY OF THE INVENTION

The process for dispersion strengthening of alloy metal can besubstantially improved by the elimination of the surface oxide build-upon the alloy metal particles by preventing the oxide surface formationor by removing the oxide build-up prior to the step of internaloxidation. The improved process comprises atomization of alloy metal andeliminating the surface oxide by preventing oxide build-up or byremoving the oxide prior to the step of internal oxidation and formingconsolidated dispersion strengthened metals.

DETAILED DESCRIPTION OF THE INVENTION

In practicing this invention, the powdered alloy comprising a relativelynoble matrix metal and a solute metal is produced by conventionaltechniques such as melting the metal under inert or reducing conditionsand thereafter comminuting the alloy by atomization to form aparticulate alloy having an average particle size of less than about 300microns. Water atomization of molten metal alloys is shown in U.S. Pat.No. 2,956,304 wherein metal particles are produced at particularly smallparticle sizes less than about 100 mesh. Water atomization similarlycauses considerable surface oxidation of alloy particles due to the hightemperatures of molten metal as well as the oxidizing characteristics ofthe water itself.

The noble matrix metal in the alloy can be defined broadly as thosemetals having a melting point of at least about 200° C. and whose oxideshave a negative free energy of formation at 25° C. of from 0 to 70kilocalories per gram atom of oxygen. Suitable alloy matrix metalsinclude, for example, iron, cobalt, nickel, copper, cadmium, thallium,germanium, tin, lead, antimony, bismuth, molybdenum, tungsten, rhenium,indium, palladium, osmium, platinum, and rhodium as more particularlyset forth in U.S. Pat. No. 3,779,714.

In any particular combination of matrix metal and solute metal in thealloy to be dispersion strengthened by internal oxidation, the matrixmetal must be relatively noble with respect to the solute metal so thatthe solute metal will be preferentially oxidized. This is achieved byselecting the solute metal such that its negative free energy of oxideformation at 25° C. is at least 60 kilocalories per gram atom of oxygengreater than the negative free energy of formation of the oxide of thematrix metal at 25° C. Such solute metals have a negative free energy ofoxide formation per gram atom of oxygen of over 80 kilocalories andgenerally over 120 kilocalories. Suitable alloy solute metals include:silicon, titanium, zirconium, aluminum, beryllium, thorium, chromium,magnesium, manganese, niobium, tantalum, and vanadium (VO), as moreparticularly set forth in U.S. Pat. No. 3,779,714. In accordance withthe process of this invention, atomized alloy particles substantiallyfree of oxide surface film are internally oxidized to formdispersion-strengthened metal.

In accordance with one aspect of this invention, atomized alloyparticles are processed to remove the oxide build-up on the particlesurface formed during atomization. The surface oxide film can bemechanically removed such as by milling, grinding or roll flaking theatomized alloy particles. Ballmilling, for example, can be used at a 4:1to 8:1 ratio of ball/metal for 2 to 8 hours. Roll flaking can be used toreduce thickness of the atomized particles as well as remove oxide filmswhich are believed to break up and/or redistribute the surface oxideover a larger surface area generated by the flaking of spherical powder.Flakes have larger surface:volume ratio than spheres of same volume.These and similar processes which deform or flatten the powder particlewould be suitable.

In accordance with a further aspect of this invention, the surface oxidefilm can be removed by chemical action such as leaching. For example,atomized copper alloy powder can be leached in dilute nitric acid,ammonium hydroxide and also in mixtures of ammonium and sodiumhydroxides.

Still, a further method of preventing surface oxide build-up on theatomized alloy particles pertains to collecting the alloy powder in adry medium and avoid wet collection mediums such as water. Drycollection within helium, for instance, prevents contact with anoxidizing substance as well as avoids steam formation within theatomization chamber. Helium is substantially better than nitrogen inthat the thermal conductivity thereof is 6.5 times that of nitrogenwhereby much faster quenching, without appreciable oxidation, can beachieved. Helium quenching enables faster quenching of the atomizedparticles, thus minimizing oxidation of the solute metal such asaluminum at the particle surface and further minimizes migration ofsolute metal from the center of the alloy particle to the particlesurface which can detrimentally deplete the alloy particle of solutemetal.

The alloy particles being substantially free of surface oxide build-upcan be internally oxidized by a variety of methods such as disclosed inU.S. Pat. Nos. 3,488,185; 3,552,954; and 3,179,515. A particularlypreferred method is shown in commonly assigned U.S. Pat. No. 3,779,714wherein 100 weight parts of alloy particles are mixed with about 0.1 to10 weight parts of oxidant. The exact proportion of oxidant mixturedepends on the solute metal to be oxidized and the concentration ofsolute metal in the alloy.

The preferred oxidant comprises an intimate mixture of heat-reduciblemetal oxide having a negative free energy of formation at 25° C. of upto about 70 kilocalories per gram atom of oxygen, and finely dividedhard, refractory metal oxide having a negative free energy of formationexceeding the negative free energy of formation of the heat-reduciblemetal oxide by at least about 60 kilocalories per gram atom of oxygen at25° C. The heat-reducible metal oxide is present in the oxidant in anamount sufficient for complete oxidation of the solute metal in thealloy. The hard, refractory oxide in the oxidant is present insubstantially the same equivalent elemental proportion as the solutemetal in the alloy, and both are of a particle size suitable fordispersion strengthening of the oxidant residue resulting from theinternal oxidation, as set forth in U.S. Pat. No. 3,779,714. Afterinternal oxidation, the oxidant residue comprises particles of in situresidue of heat-reducible metal oxide and particles of hard, refractorymetal oxide uniformly distributed therein and the residue ofheat-reducible metal oxide is intimately dispersed within the alloypowder. The dispersion-strengthened metal mixture is eventuallycoalesced and consolidated by hot-working to form a solid metalworkpiece whereby the residue of heat-reducible metal is dispersionstrengthened by the hard, refractory metal oxide and forms an integralpart of the dispersion strengthened resulting workpiece. Dispersionstrengthened metal powders are ordinarily consolidated under heat andpressure such as by extrusion at temperatures usually above about 1400°F. wherein the extrudate emerges from the extrusion press typically incylindrical bar stock which then can be cold drawn and machined to thedesired configuration of the workpiece.

The advantages of this invention wherein surface oxide build-up iseither prevented or removed from alloy particles prior to the step ofinternally oxidizing are further illustrated in the following examples.

EXAMPLE 1

A copper alloy containing 0.2% by weight alloyed aluminum was atomizedby helium and collected dry. The powder was then heat treated inaccordance with the process of U.S. Pat. No. 3,779,714 and finally hotextruded into 1/4" dia rods. Table 1 shows the room temperaturemechanical properties of this material compared with the conventionalalloy made by nitrogen atomization and water collection.

                  TABLE 1                                                         ______________________________________                                        Room Temperature Properties of Internally                                     Oxidized Cu-0.20% Al alloy. (N.sub.2 /wet vs. He/dry)                                            Tensile          Electrical                                Atomization                                                                             Hardness Strength Elongation                                                                            Conductivity                              ______________________________________                                        (gas/collection)                                                                        (R.sub.B)                                                                              (psi)    (%)     (% IACS)                                  N.sub.2 /water                                                                          58       58,000   24      91                                        He/dry    68       68,000   22      92                                        ______________________________________                                    

There is a significant improvement in room temperature tensile strengthand hardness on helium atomized and dry collected material. Theimprovement is even more dramatic at high temperatures. This isdemonstrated by the data in Table 2.

                  TABLE 2                                                         ______________________________________                                        1550° F. Stress Rupture Properties of Internally                       Oxidized Cu-0.20% Al alloy. (N.sub.2 /wet vs. He/dry)                                      100 Hour Rupture Strength                                        Atomization  at 1550° F.                                               ______________________________________                                        (gas/collection)                                                                           (psi)                                                            N.sub.2 /water                                                                             3,800                                                            He/dry       8,000                                                            ______________________________________                                    

The 100 hour rupture strength at 1550° F. was more than doubled when thealloy powder was atomized by helium and collected dry. This alsocomprises a significant improvement over the best values reported inliterature for similar composition in as extruded condition. Preston andGrant¹ report a 100 rupture stength value of 6,000 psi for internallyoxidized Cu-0.23% Al alloy. The room temperature properties reported bythem are similar to those of He/dry material.

I claim:
 1. In a process for dispersion-strengthening atomized alloyparticles having an average particle size less than about 300 microns byinternally oxidizing said alloy particles, the improvementcomprising:providing atomized .Iadd.copper/aluminum .Iaddend.alloyparticles being substantially free of oxide surface film whereby saidalloy is internally oxidized without obstruction of the oxide surfacefilm.
 2. The process of claim 1 wherein the atomized particles are drycollected within an inert gas or liquid to prevent oxide surface filmformation on the atomized particles.
 3. The process in claim 2 whereinthe atomized particles are dry collected within helium gas.
 4. Theprocess of claim 1 wherein the atomized alloy particles are subjected tomechanical working to mechanically remove the oxide surface film.
 5. Theprocess in claim 4 wherein the mechanical working is ball milling. 6.The process in claim 4 wherein the mechanical working is grinding. 7.The process in claim 4 wherein the mechanical working is flaking.
 8. Theprocess in claim 1 wherein the oxide surface film is removed by chemicalleaching.
 9. The process in claim 8 wherein the chemical leaching agentis nitric acid.
 10. The process in claim 8 wherein the chemical leachingagent is ammonium hydroxide.